.250 O'Neil Magnum

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A photo of the .250 O'Neil Magnum, featured in the April 1937 issue of American Rifleman magazine.
A photograph of the .250 O'Neil Magnum and a 200 yard grouping achieved with the cartridge. It was based on the .275 H&H Magnum, necked down to .250 caliber. Photo from from Elmer Keith's article "The .250 O'Neil Magnum" in April 1937 issue of "The American Rifleman".

The .250 O'Neil Magnum was a wildcat cartridge created around 1936 by Con Schmitt and Charles M. O'Neil, the latter being the cartridge's namesake. It was intended to compete with a notoriously accurate German cartridge, the .244 Halger. It was also meant to match the .244 Halger's high muzzle velocity. The concept went through many iterations with various cartridges being necked down, shoulder angles changed, and so on. This final cartridge was created by necking down the .275 H&H Magnum to .25 caliber.